A fluid and melt inclusion study of Fogo volcano, Cape Verde archipelago

Fogo volcano, in the Cape Verde Archipelago, is one of the most active ocean island basalts (OIBs) volcano on Earth. Previous independent studies have focused on reconstructing the magma plumbing system’s architecture, the isotopic composition of the mantle source, and the geochemistry of volcanic gases. In this dissertation, through the study of fluid (FI) and melt (MI) inclusions hosted in olivine and clinopyroxene crystals from mafic products of the recent (<120 ka) activity period of Fogo, significantly advances the previous knowledges on the magma plumbing system and isotopic composition of the mantle source. Furthermore, we have quantified the parental melt volatile contents and the volatile evolution (degassing) throughout the different levels of the plumbing system during magma ascent. In addition, our investigation extends to investigating the presence of recycled crustal carbon within the mantle source, through carbon isotopes studies.Our findings confirm the presence of a vertically elongated plumbing system with a deep-seated magma reservoir at depths of 27-36 km. This reservoir feeds the recent (<10 ka) alkali-rich magmatism of Fogo. We also identify a main magma storage zone at ̴ 13-24 km depth and a magma stagnation zone at ̴ 9-12 km depth. Furthermore, our research suggests a potential upward shift in pre-eruptive magma storage conditions since the 1951 eruption. Additionally, we highlight the volatile-rich nature of the Fogo parental melt, exhibiting elevated H2O (up to 2.73 wt.%), CO2 (up to 2.15 wt.%), S (up to 8200 ppm), Cl (up to 1400 ppm), and F (up to 2200 ppm) contents. The exceptionally high CO2 content measured in the parental melt, up to 2.15 wt.%, reflects a carbon-rich signature in the mantle source (355-414 ppm). Our findings are consistent with recent observations indicating a carbon-rich nature in alkali-rich OIB magmas worldwide (e.g., El Hierro and La Palma in Canary Islands; Piton de la Fournaise in La Réunion Island).Additionally, we have developed a carbon degassing model from mantle to crust. This model predicts a crustal signature for carbon, with a δ13C value of ̴ -0.4‰, in the primary melts (9.0 wt. % of CO2) generated through upper mantle melting at ̴ 2200 MPa (̴ 77 km). This is interpreted as indicative of mantle metasomatism by melts/fluids enriched with a recycled crustal carbon component. Finally, through noble gases analyses, we constrain the Fogo 3He/4He signature (7.14-8.44 Rc/Ra) within the typical MORB-like mantle range.